A light-emitting element using a luminescent material has features of thin shape and lightweight, high response speed, low direct-current voltage drive, and the like, and is expected to be applied to a next-generation flat panel display. A light-emitting device in which light-emitting elements are arranged in a matrix has superiority in wide viewing angle and high visibility as compared with conventional liquid crystal display devices.
The light emission mechanism of a light-emitting element is as follows: An electron injected from a cathode and a hole injected from an anode are recombined in the luminescence center in an electroluminescent layer to form a molecular exciton by applying a voltage to a pair of electrodes with the electroluminescent layer interposed therebetween, and energy is released to emit light when the molecular exciton returns to the ground state. An excited singlet state and an excited triplet state are known as an excited state, and it is believed that light can be emitted through either state.
As for such a light-emitting element, there are many problems related to materials in improving characteristics thereof. Therefore, improvement of an element structure, a development of a material, and the like are conducted in order to overcome these problems.
As one of the element structures, a light-emitting element is reported, which has a structure in which a plurality of luminescent units are stacked to be divided by a charge generation layer between an anode and a cathode that are opposed to each other in order to realize a longer lifetime when light is emitted with high luminance (Patent Document 1 and Non-Patent Document 1). The charge generation layer has the function of injecting carriers and is required to include a highly light-transmitting material.
[Patent Document 1]
Japanese Patent Laid Open Disclosure No.: 2003-45676
[Non-Patent Document 1]
Toshio Matsumoto, Takeshi Nakada, Jun Endo, Koichi Mori, Norihumi Kawamura, Akira Tokoi, and Junji Kido, IDW' 03, pp. 1285-1288